Differential pressure (DP) flow metres are the oldest and most widely deployed flow measurement technology in industrial processes. From power stations to chemical plants to water treatment facilities, DP metres dominate large-pipe applications—not because they are the most accurate, but because they are economical, robust, and universally understood.
This guide explains how DP metres work, the major types, and how to select and install them correctly according to ISO 5167 standards.
How Differential Pressure Flow Metres Work
The Physics: Bernoulli's Equation
Differential pressure metres exploit Bernoulli's equation—a principle from fluid mechanics stating that as fluid velocity increases, pressure decreases, and vice versa.
- A flow restriction (orifice plate, venturi throat, nozzle) is placed in the pipe
- Upstream of the restriction, fluid pressure is P₁ and velocity is V₁
- At the restriction, the fluid accelerates to velocity V₂, and pressure drops to P₂
- The differential pressure (ΔP = P₁ − P₂) is proportional to the square of the flow velocity: ΔP ∝ V²
- A differential pressure transmitter measures ΔP and outputs a signal proportional to volumetric flow rate
Key relationship: Volumetric flow Q = C × A × √(2×g×ΔP / ρ), where C is the discharge coefficient, A is the restriction area, g is gravitational acceleration, and ρ is fluid density.
Measurement Uncertainty
Because flow is proportional to the square root of ΔP, measurement uncertainty is amplified at low flow rates. For example:
- At 100% flow with DP of 100 mbar: ±3% DP error = ±1.5% flow error
- At 50% flow with DP of 25 mbar: ±3% DP error = ±3% flow error (double the relative error)
This non-linear relationship is a defining limitation of DP metres at low flows and motivates the importance of turndown ratios.
Types of Differential Pressure Metres
1. Orifice Plate (Most Common)
A thin flat disc with a concentric hole, inserted between two pipe flanges.
- Design: Typical hole diameter = 0.5 to 0.8 × pipe diameter
- Pressure loss: 40–70% of differential pressure is permanent (energy dissipated as heat)
- Accuracy: ±1.0% to ±3.0% (depends on Reynolds number and installation)
- Cost: £100–£500 (plate itself is cheap; flanges and transmitter cost more)
- Advantages: Lowest capital cost, well-established (ISO 5167), no moving parts, works with any fluid
- Disadvantages: High permanent pressure loss, sensitive to flow profile, erosion damage from dirty fluids, requires large straight pipe runs (20–40D)
- Applications: Steam measurement, gas flow, water distribution, power plant feed water
2. Venturi Tube
A specially shaped pipe section with a converging inlet cone, a throat, and a diverging recovery cone.
- Design: Smooth convergence over 0.5 pipe diameters; gradual recovery cone (4–8 pipe diameters)
- Pressure loss: Only 5–20% of differential pressure is permanent (remainder recovers in recovery cone)
- Accuracy: ±0.5% to ±1.5% (better than orifice plates)
- Cost: £500–£5,000 (specialty item; expensive for large diameters)
- Advantages: Lower permanent pressure loss; better accuracy; self-cleaning design tolerates dirty fluids; excellent for steam and high-velocity gas
- Disadvantages: Higher capital cost; requires longer installation (full-size pipe section); less versatile than orifice plates
- Applications: Steam measurement in power plants, high-velocity gas flows, custody transfer of natural gas
3. Flow Nozzle
A streamlined nozzle-shaped insert that fits between two pipe flanges, intermediate between orifice plate and venturi in design.
- Design: Rounded convergent inlet; no recovery section (creates higher permanent loss than venturi but lower than orifice)
- Pressure loss: 25–60% of differential pressure is permanent
- Accuracy: ±0.8% to ±2.0%
- Cost: £300–£2,000
- Advantages: Better erosion resistance than orifice plates; smoother profile reduces cavitation risk; smaller than venturi
- Disadvantages: More expensive than orifice plate; still significant pressure loss
- Applications: Steam measurement, erosive or high-velocity flows, space-constrained installations
4. V-Cone
A cone inserted into the pipe centre from the upstream wall, creating annular (ring-shaped) flow restriction.
- Pressure loss: 35–55% permanent loss
- Accuracy: ±0.5% to ±1.5%
- Advantages: Insensitive to velocity profile (works well in pipes with bends upstream); tolerates some upstream disturbance
- Disadvantages: Higher cost; requires machined pipe boss connection
- Applications: Retrofit installations where straight pipe runs unavailable; variable-profile flows
5. Wedge Meters
A wedge-shaped blockage inserted into the pipe, creating a small gap as the restriction.
- Accuracy: ±1.0% to ±2.5%
- Advantages: Good for slurries and dirty fluids (wedge design sheds particulates); low blockage risk
- Disadvantages: Less accurate than orifice plates; specialist manufacturer required
- Applications: Slurry metering, sewage, pulp and paper flow measurement
6. Averaging Pitot Tube
Multiple small tubes sample velocity at different points across the pipe, averaging the profile.
- Accuracy: ±2% to ±5%
- Advantages: Minimal pressure loss (<0.2 bar); can be installed in existing pipes via threading; less sensitive to velocity profile than single-point pitot
- Disadvantages: Lower accuracy; plugging risk in dirty fluids; less common
- Applications: Retrofit duct flow measurement, low-pressure gas, monitoring only (not custody transfer)
Differential Pressure Metre Specifications
Accuracy
Typical range: ±1.0% to ±3.0% (orifice plates), ±0.5% to ±1.5% (venturi tubes).
Accuracy depends on:
- Reynolds number (transition from laminar to turbulent flow)
- Upstream flow profile (bends, valves, fittings upstream distort profile)
- Differential pressure transmitter calibration and stability
- Beta ratio (restriction diameter / pipe diameter)
Turndown Ratio
Typical: 3:1 to 5:1 (limited due to square-root relationship).
A DP metre sized for 100 mbar maximum ΔP at full flow will read only ±3% at 1/3 flow (where ΔP = 11 mbar). To measure down to 10% flow accurately, you would need to reduce maximum ΔP design, which reduces the turndown at low pressure systems.
Pressure Loss
Orifice plate: 40–70% of ΔP is permanent. For a ΔP of 100 mbar, permanent loss = 40–70 mbar (unrecoverable).
Venturi tube: 5–20% of ΔP is permanent. For a ΔP of 100 mbar, permanent loss = 5–20 mbar (recovery cone recovers most energy).
In high-throughput applications (water distribution, power plants), this pressure loss translates directly to pumping cost. Venturi tubes reduce operational cost significantly over orifice plates.
Temperature and Pressure Range
Standard materials: Carbon steel (most common), stainless steel (corrosion resistance).
Temperature range: −40°C to +120°C (dependent on material and seal design).
Pressure rating: Flanged connections rated to ISO 1217 or ANSI; typically 16 bar (PN16) or 25 bar (PN25) in UK applications.
Fluid Compatibility
DP metres work with any fluid—liquids, gases, steam, slurries, corrosive chemicals. Material selection (carbon steel vs. stainless steel vs. exotic alloys) determines compatibility, not the measurement principle.
ISO 5167 Standard Compliance
What is ISO 5167?
ISO 5167 is the international standard specifying design, installation, and accuracy of differential pressure flow measurement devices. Published jointly by ISO and IEC, it provides:
- Equations for flow calculation based on ΔP, discharge coefficient (C), and piping geometry
- Discharge coefficient values for different restriction types and Reynolds number ranges
- Requirements for straight pipe runs (upstream and downstream)
- Pressure tap locations (corner taps, flange taps, pipe taps)
- Beta ratio limits (restriction diameter / pipe diameter)
- Reynolds number constraints for accuracy claims
Straight Pipe Run Requirements
ISO 5167 specifies upstream and downstream straight runs to ensure a stable, predictable velocity profile:
- Orifice plate: 20D upstream, 5D downstream (D = pipe diameter)
- Venturi tube: 5D upstream, 4D downstream (due to self-cleaning recovery section)
- Flow nozzle: 10D upstream, 5D downstream
Non-compliance with these requirements introduces 2–5% measurement error. This is why retrofit installations often underperform; existing pipe geometry prevents achieving the required straight runs.
Beta Ratio (β)
Defined as β = d / D, where d is the restriction diameter and D is the pipe diameter.
- Typical range: 0.4 to 0.8
- Higher β (e.g., 0.75): Lower pressure loss, lower ΔP signal (requires more sensitive transmitter)
- Lower β (e.g., 0.4): Higher ΔP signal, easier to measure, but more permanent pressure loss
Applications of Differential Pressure Metres
1. Power Generation: Steam Measurement
Power stations use orifice plates and venturi tubes to measure superheated steam flow to turbines. Steam's compressibility and high velocity make DP metres ideal (electromagnetic would not work, turbine metres would erode).
2. Water Treatment and Distribution
Municipalities measure potable water, reclaimed water, and process water using orifice plates and venturi tubes. For large diameters (6–12 inches), the DP metre's cost-effectiveness is compelling.
3. Gas Flow in Pipelines and Plants
Natural gas distribution networks, biogas facilities, and industrial gas systems use orifice plates (simple, cheap) or venturi tubes (for higher-accuracy custody transfer) to measure volume flow.
4. Chemical Plants and Refineries
Process monitoring of crude feed, steam injection, and gas recycling uses DP metres. For high-temperature steam-oil mixtures, DP metres are often the only viable option.
5. HVAC and Duct Flow
Building automation systems use orifice plates or averaging pitot tubes in ductwork to balance air-handling unit flows and ensure commissioning targets are met.
Advantages of Differential Pressure Metres
No Moving Parts
The restriction element (plate, tube, nozzle) is passive. No gears, vanes, or rotors to wear out. Service life: 20+ years with minimal maintenance.
Lowest Cost for Large Pipes
For pipes 6+ inches, an orifice plate installation (plate + flanges + DP transmitter) can cost £500–£1,500 total, making it dramatically cheaper than Coriolis or electromagnetic alternatives at large diameters.
Universally Understood
Every process engineer understands DP metres. Troubleshooting, calibration, and replacement are straightforward. Equipment and spares are readily available globally.
Well-Established Standards
ISO 5167 provides a proven methodology. Regulatory authorities (EPA, UK HSE) accept DP metres for custody transfer if properly installed per standard.
Works with Any Fluid
Steam, gas, liquid, slurry—DP metres work universally. No conductivity, viscosity, or density constraints.
Retrofit Capability
Orifice plates can be retrofitted into existing flanged pipe sections without major disruption.
Disadvantages and Limitations
High Permanent Pressure Loss (Orifice)
Orifice plates dissipate 40–70% of the differential pressure as unrecoverable heat, increasing pumping cost over the metre's lifetime. For a 1,000 L/min water flow with 100 mbar ΔP, this pressure loss can add £500–£1,000 annually to electrical costs (depending on electricity rates).
Limited Turndown Ratio
The square-root relationship means DP metres cannot measure accurate flows over a wide range. A metre sized for 100% flow will read poorly at 10% flow. This forces operators to install multiple metres for different flow ranges or accept accuracy loss.
Accuracy Degradation at Low Flows
At flows below 30% of design, DP metre accuracy drops significantly (±3% to ±5%). For applications with variable flow (demand-driven systems), this is a limitation.
Sensitivity to Upstream Disturbances
Bends, valves, or equipment upstream distort flow profile, causing DP metre errors unless sufficient straight pipe is provided. Retrofit installations often suffer from this limitation.
Erosion and Plugging Risk
Dirty or slurry-laden fluids erode orifice plate edges over time, and particulates can lodge at the restriction, creating measurement bias. Venturi tubes and wedge metres are more erosion-tolerant.
Not Suitable for Very Large Pipes (>24 inches)
At very large diameters, ΔP metres become unwieldy. Electromagnetic or ultrasonic metres become more practical.
Complex Sizing and Calculation
Proper DP metre sizing requires iterative calculation of beta ratio, Reynolds number, and discharge coefficient. This is not a simple plug-and-play selection.
Differential Pressure vs Other Metres: Quick Comparison
Choose DP if:
- Pipe diameter >6 inches (cost advantage)
- Fluid is gas or steam (no moving parts to erode)
- Accuracy ±1–3% is acceptable
- Capital cost is a priority
- Maintenance-free operation is desired (>20 years service life)
Choose alternatives if:
- Accuracy >±0.5% required (use Coriolis or PD)
- Flow variability is 10–100% (use electromagnetic for >20:1 turndown)
- Pressure loss cannot be tolerated (use electromagnetic or ultrasonic)
- Pipe diameter <2 inches (Coriolis or PD more practical)
Sizing and Installation Best Practices
Step 1: Determine Required ΔP Range
Choose a maximum ΔP between 50–300 mbar (higher ΔP = better signal-to-noise ratio, but more pressure loss). Most industrial applications target 100 mbar at full flow.
Step 2: Calculate Beta Ratio
Use ISO 5167 equations to derive the beta ratio that produces your target ΔP at full flow.
Step 3: Verify Straight Pipe Availability
Measure or confirm available upstream and downstream straight pipe. Non-compliance introduces 2–5% error; plan accordingly or select a more forgiving design (venturi, V-cone).
Step 4: Choose DP Transmitter
Select a transmitter with range 0–150 mbar (for 0–100 mbar of your DP) to maximise accuracy. Ensure transmitter isolation valves for safe maintenance.
Step 5: Install Pressure Taps Correctly
Follow ISO 5167 tap locations (corner taps for orifice plates are most common). Incorrect tap location introduces calibration error.
Step 6: Commission and Verify
Compare calculated flow from DP reading against independent measurement (weigh tank, bucket timing, ultrasonic metre). Variance >5% indicates installation or sizing error.
Cost Considerations
Typical Pricing (2026)
- Orifice plate complete kit (1-inch): £200–£500
- Orifice plate complete kit (6-inch): £500–£1,500
- Venturi tube (2-inch): £1,000–£2,000
- Venturi tube (6-inch): £2,500–£5,000
- DP transmitter (0–100 mbar): £300–£800
Total Cost of Ownership
- Plate replacement every 5–10 years (erosion or corrosion): £150–£400
- DP transmitter calibration every 2 years: £100–£300
- Operational cost (pressure loss energy): £500–£2,000 annually for large pipes